Iron or steel, when heated to above 900 °F (480 °C), glows with a red color, the color being indicative of the temperature in circumstances of low-technology (today we understand this association of temperature with 'color' as being due to black body radiation). Good quality iron or steel at and above this temperature becomes increasingly malleable and plastic. Iron or steel having too much sulfur, on the other hand, becomes crumbly and unworkable (by traditional blacksmithing techniques).
The term "Hot-shortness" is also used to describe a steel which is brittle at high temperatures and cannot be worked. The element Sulfur (S) combines with Iron (Fe) to form the compound FeS - Iron Sulfide. This compound concentrates at the grain boundaries of the steel and is a lower melting point than the steel itself. When the steel is heated up and worked, the mechanical energy added to the workpiece increases the temperature further. The Iron Sulfide (FeS) begins to melt, and the steel starts to separate at the grain boundaries. Steelmakers add Manganese (Mn) to the steel when it is produced, to form Manganese Sulfide (MnS). Manganese sulfide inclusions have a higher melting point and do not concentrate at the grain boundaries. Thus, when the steel is later heated up and worked, the melting at the grain boundaries does not occur.